Add optional GitHub API authentication headers

If the `GITHUB_TOKEN` is defined in the Ansible controller's
environment it is used to set the GitHub REST API authentication header.
This is they passed to all `uri` modules tasks which interact with the
GitHub REST API. If the `GITHUB_TOKEN` is not set, the authentication
header is not used.

Fixes #19

roles/git/tasks/Windows-installer.yaml

@@ -2,6 +2,7 @@
 - name: get json containing all releases
   win_uri:
     url: 'https://api.github.com/repos/git-for-windows/git/releases/tags/v{{git_version}}.windows.1'
+    headers: '{{github_auth_headers}}'
     return_content: true
   register: git_release